API: Add filtering using field lookups to UnitResource

Units are only listed when a filter is provided

Also expand UnitResource docs to document this feature
API: Add UserResource resource
API: Add UserObjectsOnlyAuthorization

This will allow to forbid access to User resources that belong to other users
API: Add statistics to UserResource
NEEDS WORK    Docs: Add docs for API UserResource
UNDO    API: Add sample pootle-cli client for API

It is meant for learning purposes
UNDO    IGNORE THIS: misc comments
UNDO  not working tests with pootle-cli